Evidence for Enhanced Electrostatic Interactions in Lamellar Monolayer Systems

Abstract

Electrostatics, magnetostatics. PACS. 61.30E Experimental determinations of smectic, nematic, cholesteric, and lyotropic structures.Abstract. The interlamellar spacing as a function of the surfactant concentration in a lamellar monolayer system containing a ionic surfactant was studied. It is demonstrated that even at large interlamellar spacings (6-14 nm) relative to the Debye lengths (order 0.7 nm), electrostatic repulsion still plays a significant role which is attributed to a coupling with monolayer undulations.Systems consisting of water, oil and one or more amphiphiles may order themselves into a stack of undulating monolayers or bilayers. Over the last decade it has become clear that undulations may strongly renormalize direct interactions arising from intermolecular forces [1][2][3][4]. Thermal undulations are predicted to lead to a much slower decay of the electrostatic repulsion between charged bilayer membranes than expected from classical double-layer theory [5][6][7][8]. Supporting experimental evidence for such an effect can be obtained from the distance dependence of the interaction between charged membranes [9]. Moreover, Odijk [10] deduced from his self-consistent theory of undulation-enhanced interactions a melting rule for a lamellar phase coexisting with an isotropic phase which is in semi-quantitative agreement with experimental results of Dubois and Zemb [11]. The aim of the work presented here is to provide experimental evidence for enhanced electrostatic repulsion in single-phase fluctuating monolayer systems in which the interlamellar spacing is measured as a function of the monolayer volume fraction. The effective salt concentration in these systems is shown to depend upon the total area of the charged monolayer at the oil-brine interface. We present a simple method to control the effective salt concentration independent of the monolayer volume fraction and show that even at relatively high effective salt concentrations (up to 0.3 M), electrostatic repulsion still plays a significant role. We first summarize some theoretical aspects of charged undulating membrane systems that are relevant for the work presented here. In a fluctuating membrane system thermal undulations tend to decrease the projected area while the total area A is conserved. The first-order correction to the projected area Aproi of a membrane caused by thermal (*) Present address:

The ultrastructure of fibronectin fibers pulled from a protein monolayer at the air-liquid interface and the mechanism of the sheet-to-fiber transition

“…To investigate the factors that regulate fiber architecture, we first looked at electrostatic interactions, since it is known from the literature that they play an important role in stabilizing various surfactant, lipid and clay lamellar systems [43,44] . For this purpose, we analyzed the structure of fibers pulled out of solutions of different ionic strengths.…”